The objective of this project is to investigate the relationship between opioid receptor regulation and the pharmacodynamics of opioid agonists. To accomplish this goal the project will use a multi-faceted approach employing in vivo and in vitro functional assays, opioid receptor binding studies and studies on opioid receptor gene expression. Understanding the mechanisms by which opioid drugs produce their effects is important because of their role as abused drugs and their clinical utility. The information resulting from these studies will provide insights into opioid analgesia, tolerance and dependence and may aid in developing new strategies for pain management and treatment of opioid drug abuse. The proposed studies will examine changes in opioid receptor mRNA in mouse brain, spinal cord and striatum. In parallel, changes in opioid receptor binding and opioid potency in in vivo and in vitro dose-response experiments will be determined. Chronic and acute opioid agonist and antagonist treatments have been-shown to alter opioid analgesic potency and opioid receptor density. Genetic factors, in the form of mouse strains which are opioid receptor deficient, can also impact on opioid agonist potency. In addition, gonadal and adrenal hormones can modify opioid potency. These hormonal, genetic and drug-induced changes in opioid potency and receptor density will be studied to determine the role of changes in opioid receptor gene expression. The proposed parallel studies on opioid receptor mRNA, receptor binding and function will help to define the relationships between function, receptor density and gene expression. The coupling of opioid receptors to intracellular events will also be investigated. Opioid receptor activation is modulated via pertussis toxin sensitive guanine nucleotide binding proteins (G-proteins). The effect of in vivo pertussis toxin treatment on opioid potency and G-proteins will be explored. Recent studies show that opioid agonist intrinsic efficacy can directly modify the regulation of opioid receptors and the development of tolerance and that opioid tolerance is contingent on variations in the dosing protocol. Therefore, studies will examine the relationship between dosing parameters, opioid agonist intrinsic efficacy and tolerance. Intrinsic efficacy for a series of mu opioid agonists will be estimated with an irreversible mu opioid receptor antagonist. Finally, antagonists at N-methyl D-aspartate (NMDA) receptors have been shown to limit the development of tolerance to morphine. The relationship between NMDA receptor antagonists and opioid tolerance will be studied using several different opioid agonists and opioid receptor deficient mouse strains. Information gathered by this project will impact on our understanding of factors that are significant in modulating dynamic changes in the opioid system. These studies will yield information on the regulation of opioid analgesic potency using behavioral, receptor binding and molecular pharmacologic approaches.